Abstract:

The average catch per unit of swept area (CPUE)
from bottom trawl surveys by research vessels are
used as an index of abundance for the demersal fish
resources of the Thai waters of the Andaman Sea.
The surveys are carried out under the assumption
of constant swept area and catch efficiency.
However, preliminary investigations showed that the
geometry varied by depth and based on other
studies it is likely that this may affect CPUE.
To reduce the variability in trawl geometry by depth,
the constrictor rope technique was applied. The
length of the rope (6 m) and its position (between
the warps; 100 ahead of the trawl doors) were
based on trials without rope at the shallowest depth
trawled during the routine surveys (20 m).
Using the constrictor rope at deeper depths gave
similar trawl geometry as obtained at the reference
depth of 20 m. When the trawl was fished with the
constrictor rope 75 m depth, the mean door
distance was reduced by 23% compared to the door
distance obtained for hauls without a constrictor
rope. Similarly, wing distance was reduced by 18%
and the angle of the sweep/bridles was reduced
from 20.3 to 15 degrees (26% reduction). The
vertical trawl opening was increased marginally
(0.12 m; 6.5 %)
Despite the difference in trawl geometry, catch
rates and catch composition did not differ
significantly between the hauls made with and
without the constrictor rope. The reason for this is
unclear, but it is suggested that the larger area
swept when no rope is used is offset by increased
escape below the fishing line due to the trawl being
overspread.
Observations by underwater video cameras in the
net mouth area during hauls at shallow depth and
without constrictor rope, indicated proper bottom
contact of the fishing line. It is therefore suggested
that the trawl will have similar bottom contact at all
depths when the constrictor rope is used. However,
this needs to be verified. Observations of fish
showed that fish took up position slightly in front of
the fishing line, swimming in the direction of towing.
Except for a few rays, no fish were observed to
escape below the fishing line. Observations of
trawl's fish lock showed that this did not function as
intended as fish were observed to swim forward out
of the codend during haul back of the trawl.
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Sections of gillnet panels mounted to the outside of
the top panel of the trawl, showed only marked
escape during one haul and only close to the
codend. The catch is this haul was approximately an
order of magnitude higher than in the remaining
hauls, suggesting a density dependent escape.